1. Field of the Invention
The present invention relates to a load-drive controller, and to a load driving apparatus provided with a control circuit for an H bridge circuit suitable for driving a reactance load.
2. Description of the Related Art
Drive of a stepping motor is controlled by controlling a current (hereinafter referred to as “load current”) applied to a load such as a coil. The load is connected to an H bridge type load driving circuit (hereinafter referred to as “H bridge circuit”), and the load current is increased or decreased by turning on or off transistors provided in the H bridge circuit for supplying a load current.
A load-drive controller includes a voltage source for setting a current to have a reference value, a comparator for comparing the current setting voltage generated by the voltage source and a voltage obtained through a current detecting resistor, a PWM control unit, an oscillator for generating a PWM waveform signal, a gate driver for driving output transistors, and the H bridge circuit for driving a load. The load current is converted into a voltage through the current detecting resistor.
For example, in a conventional load-drive controller disclosed in Japanese Patent Laid-Open Publication No. 9-219995, an H bridge circuit includes four transistors (hereinafter referred to as M1, M2, M3, and M4) and repeats ON/OFF operations of the transistors. Specifically, when receiving a command in which an electrically conducting direction becomes M1→load M→M4, M1 is always turned on, M4 is repeatedly turned on and off, M3 is always turned off, and M2 is repeatedly turned on and off. When the electrically conducting direction is inverted, M2 is always turned on, M3 is repeatedly turned on and off, M4 is always turned off, and M1 is repeatedly turned on and off.
In this conventional current control method, a recovery current flows at the moment M4 is turned on in an electrically conducting mode. Therefore, M4 is turned on to increase the load current while the command of the comparator is neglected during an enforcement conducting time, and the comparator compares a voltage detected by a current detecting resistor (hereinafter referred to as “detection voltage”) and a current setting voltage (hereinafter referred to as “reference voltage”, too), and comparison result is reset at a time the detection voltage is matched with the current setting voltage. Thus, the step of the control process enters a diode regenerative mode in which M4 is turned off, M1 is turned on, and M2 is turned off. After a predetermined time elapsed, M2 is turned on so that the step of the process enters a synchronous rectification mode, and thereafter M2 is turned off again and the step of process enters the diode regenerative mode. After the electrically conducting mode is ended, the load current is decayed, and the step of the process enters the electrically conducting mode at predetermined intervals to increase the load current. By repeating these steps of the process, the peak value of the load current flowing through the load is controlled to be constant.
Next, a conventional method of keeping the peak value of the load current constant is described with reference to FIGS. 7 to 9. FIG. 7 shows a conventional relationship between the setting current and the load current, FIG. 8 shows a changing state of a conventional setting current in a micro step manner; and FIGS. 9A and 9B show a conventional relationship between the setting current and the load current, wherein FIG. 9A shows a state of the load current following the setting current, and FIG. 9B shows a timing chart of the setting voltage. In FIG. 7, an interval (a) corresponds to a period of the electrically conducting mode, intervals (b) and (d) correspond to a period of the diode regenerative mode, and an interval (c) corresponds to a period of the synchronous rectification mode.
In a transition from the diode regenerative mode to the electrically conducting mode, when M4 is turned on in a state in which a forward current flows through a parasitic diode of M2 in the diode regenerative mode, a backward voltage is applied for a moment to the parasitic diode of M2 to flow a backward current (recovery current) through the parasitic diode of M2. If the recovery current flows through the current detecting resistor, there arises a trouble such that the comparator is inverted and the mode transition is made from the electrically conducting mode to the diode regenerative mode although the load current does not reach the setting current. Therefore, in order to prevent such a mistake in detection, a countermeasure called an enforcement electrically conducting mode is taken to disable the result of the comparison between the load current and the setting current for a predetermined time after the transition to the electrically conducting mode. Thus, a reset signal of the comparator is neglected during the electrically enforcement conducting mode.
Meanwhile, for the purpose of silencing a stepping motor, a coil current is finely cut to form a pseudo-sine wave in a micro step drive. As shown in FIG. 8, the setting current is changed in stepwise, and the current flowing through the load is controlled like the pseudo-sine wave. In the case where the load is a stepping motor, a micro step wave is formed to realize low vibration and low noise.
In the micro step wave control shown in FIG. 8 of the conventional load-drive controller, when the setting current is changed in the current increase duration, the load current rapidly reaches the setting current by the process of the electrically conducting mode. However, in the current decrease duration, since the current decay is low-speed decay caused by diode regeneration or synchronous rectification regeneration, the decay time is determined based on a time constant of the inductance load. In addition, as described above, because the electrically enforcement conducting operation is performed at predetermined periods of the PWM control, it takes a longer time to cause the load current to reach the setting current.
In the setting current decrease duration, the current decay of the PWM control is the low-speed decay as shown in FIGS. 9A and 9B when the setting current is decreased, and it takes a long time to reach the setting current because of the existence of the electrically enforcement conducting time. Referring to FIG. 9B, the reference voltage having a certain low level is supplied in a first predetermined duration (T1), a high-level reference voltage is maintained in a second predetermined duration (T2), and the original low-level reference voltage is maintained in a third predetermined duration (T3). Referring to FIG. 9A, the load current is changed by following the reference voltage (corresponding to “setting current”). On the other hand, the detection voltage from the H bridge circuit is decreased by following the reference voltage (setting current), and the detection voltage is gently changed in the stepwise manner in the third duration T3. Therefore, there is a problem that follow-up capability of the load current to the setting current is deteriorated.